MSU Extension

Introduction

Grape production is increasing in many viticultural areas as consumer demand for wine, grape juice products and table grapes increases. Competition for grapes among processors and marketers is driving the planting of new vineyards. Presuming a reliable market for grapes has been identified, economics is the very first matter to be resolved when planting a new vineyard. Will a new vineyard be profitable? Several publications (Bordelon, 1997; Cross and Casteel, 1992; Kelsey et al., 1989; Varden and Wolfe, 1994; Walker, 1995) can guide individuals in assessing the profitability of a vineyard. If the economics are favorable, then one can begin to establish a vineyard. This publication and its companion, Extension bulletin E-2645, "Vineyard Establishment II - Planting and Early Care of Vineyards," are intended to assist in that process.

I. Selecting a Vineyard Site

Grapevines are relatively easy to grow and can live a very long time. Some Michigan vineyards are more than a hundred years old. The commercial grower's goal, however, is not merely vine survival but production of quality grapes at a profit. The first and most crucial step to achieve that goal when planting a new vineyard is selecting a suitable site.

The climate of a vineyard often is discussed at three levels (Geiger, 1957). The macroclimate of a vineyard is the large-scale or regional climate, which is influenced by geographic location (latitude) and proximity to large, climate-moderating bodies of water. Proximity to the Great Lakes, especially Lake Michigan, results in an increase in cloudiness downwind, which in turn moderates daily temperatures — i.e., daily maximum temperatures in a lake-modified climate are lower and daily minima are generally higher. Therefore, the suitability of a given location for grape production in Michigan generally decreases as one moves inland. Because of the prevailing westerly winds, the area of lake-modified climate is much wider on the western side of the state near Lake Michigan than along Lake Huron and Lake Erie on the east side of the state.

The mesoclimate is the local climate of a specific vineyard site, which is influenced by the topographic factors of elevation, slope and aspect (direction of slope) as well as close proximity to temperature-moderating bodies of water.

The microclimate is the climate within and around the vines themselves. This influences important vineyard characteristics such as how well the leaves and fruit are exposed to sunlight, what temperatures the fruit experiences through the day, how long vines remain wet and susceptible to disease infection after a rain, etc.

When a vineyard site is chosen, attention is first given to its macroclimate and then to its mesoclimate. Growers should use the information presented here together with soil surveys, local weather data and local expertise to evaluate the macroclimate and mesoclimate characteristics of a particular vineyard site.

Winter Minimum Temperatures

The most important characteristic of a site for commercial grape production in a cool climate such as Michigan's is the extent and frequency of low winter temperatures. Grape varieties have a genetic limitation for tolerating low winter temperatures. They may be placed into hardiness categories (Table 1), which designate temperatures at which significant injury to vines begins. Very cold-tender varieties may experience significant winter injury at temperatures as high as 20 degrees F (Kissler, 1983) and are not suited for cool-climate locations such as Michigan. Therefore, this discussion focuses on the selection of sites for cold-tender or hardier varieties (Table 1) that do not sustain significant winter injury until -5 degrees F or lower.

Though vine tissues have a genetic limitation for tolerating low winter temperatures, the level of this tolerance is influenced by the rate of temperature drop, cultural practices influencing maturation of vine growth in the previous growing season, cropping history, time in the winter period, potassium nutrition and soil moisture conditions of the vineyard site. Moreover, portions of a vine vary in their hardiness. For example, fruiting buds may be extensively damaged by a low-temperature episode (Fig. 1) while cane and trunk tissues remain healthy. On the other hand, rapid drops in temperatures may injure trunk tissues without significantly affecting bud tissues. Therefore, the nature and extent of winter injury are not entirely predictable for any given variety-site-weather combination.

For example, when vines of cold-tender varieties experience -5 degrees F, they may not die or even be unproductive the following growing season. Climatic conditions prior to a -5 degrees F episode may acclimate cold-tender vines so they experience little injury; cultural practices applied by a grower may also compensate for moderate levels of winter injury. Nevertheless, the risk of unmanageable injury to coldtender varieties becomes greater as temperatures dip lower. Therefore, when vines of cold-tender varieties experience -15 to -20 degrees F, their productivity will often below the following growing season. Vine survival itself may be jeopardized if the grower doesn't employ special cultural practices. For this reason, a knowledge of the frequency of temperatures of -5 degrees F and lower helps to define the potential of a site for grape production as well as its suitability for varieties within hardiness categories (Table 1).

Low winter temperatures may threaten vine survival itself, but most often the major concern is for the long-term reliability of production. Therefore, it is useful to ask, "How many years out of 10 can one expect highly productive vines with the anticipated levels of winter injury for a specific vineyard site/grape variety combination?" Winter minimum temperature frequency data to address that question for several locations in Michigan (Fig. 2) are listed in Table 2 (see page 7).

Raw winter minimum temperature data for all areas of the United States may be obtained from the National Climatic Data Center in Asheville, N.C. (phone: 704- 271-4800). The fee often will be modest, depending on the extent of the data requested. Extension personnel in other states may also be good sources of winter minimum temperatures like those presented in Table 2 for Michigan. Winter minimum-frequency data derived from raw temperature data represent the general climate, or so-called macroclimate, of a region and integrate large-scale climatological factors such as jet stream location and lake-enhanced cloudiness. Because extreme minimum temperatures usually are associated with clear, calm conditions, mesoclimatic features become important contributing factors in describing the low-temperature climate of a given site.